Very common fault,especially with leather seats.
It is almost certainly caused by a brake in the thin heater wiring in the seat base pad,but there is also another element in the back...these are both wired together so if 1 section fails the entire heater doesnt work.
Check the fuses first as it may be a simple fix...next check to see you are getting power to the seat elements with a meter...you can also remove the seat back plastic panel and inside you will find a plug connecting the base and back,so should be able to atleast establish where the fault lies.
Then the tricky bit.....if it is fabric covered they are a lot easier and you should be able to slide another seat pad in...if it is leather which seems to be the majority you then have a much bigger job.
I have a similar problem with my drivers seat which is the sports Recaro leather type..my plan is to get a upholsterer to dismantle the leather pads and fit a retro set of waeco after market elements in,as general opinion is the VX type are prone to problems,and are also not cheap.I have held fire on my project at present as £££,s are short ,although it would be the right time of the year to have them working.
Best of luck,but it can be fixed ....at a price ?